The present invention relates, in general, to semiconductor devices and processing, and more particularly, to a monolithic pressure sensor integrated with CMOS circuitry.
In the past, the semiconductor industry has formed capacitive pressure sensors which are compatible with CMOS circuit elements and which permit integration within a CMOS circuit. Examples of such pressure sensors are given in U.S. Pat. Nos. 5,321,989 and 5,431,057 both of which issued under Gunther Zimmer et al, and also in a paper H. Dudaicevs et al, xe2x80x9cA Fully Integrated Surface Micromachined Pressure Sensor with Low Temperature Dependence,xe2x80x9d 8th Int. Conf. On Solid State Sensors and Actuators and Eurosensors IX, June 25-29, pp. 616-619, all of which are hereby incorporated herein by reference.
Typically, the pressure sensor has a fixed electrode which is formed as a doped portion within the substrate underlying the sensor diaphragm. Typically, the electrode area is doped by an implant that is performed at the same time as the source and drain implants of CMOS transistors that are formed within the same substrate. Thereafter, the (poly) silicon for the diaphragm is formed overlying the diffused electrode area. Subsequently, the polysilicon is implanted and annealed to dope the polysilicon. This annealing operation affects the source and drain implants of the CMOS devices and changes the characteristics of the resulting transistors. Consequently, the characteristics of the resulting CMOS transistors differ from the originally desired characteristics.
Accordingly, it is desirable to have a method of forming an integrated pressure sensor with CMOS transistors that does not detrimentally affect the CMOS transistor characteristics.